Method for controlling the starting of an AC induction motor

ABSTRACT

A method is provided for controlling the starting of an AC induction motor with a soft starter. The windings are connected in a delta configuration and each winding has a corresponding thyristor switch connected in series therewith such that each winding and thyristor switch combination is connected between two corresponding terminals of the AC induction motor. Alpha control of the thyristor switches is utilized to start the AC induction motor and bring the AC induction motor to near full operating speed. As the AC induction motor nears full operating speed, gamma control of the thyristor switches is utilized to bring the AC induction motor to full operating speed. With the AC induction motor at full operating speed, bypass contactors, in parallel across corresponding thyristor switches, are closed and the AC induction motor operates in a conventional manner.

FIELD OF INVENTION

This invention relates to motor controls, and in particular, to a method of controlling the starting of an AC induction motor with a soft starter.

BACKGROUND AND SUMMARY OF THE INVENTION

There are two basic approaches for controlling the starting, stopping and speed of an AC induction motor. In a first approach, an adjustable frequency controller is interconnected to the AC induction motor. The adjustable frequency controller is comprised of an inverter which uses solid state switches to convert DC power to stepped waveform AC power. A waveform generator produces switching signals for the inverter under control of a microprocessor. While adjustable frequency controllers efficiently control the motor speed and the energy used by an AC induction motor, use of such types of controllers may be cost prohibitive. Further, since many applications of AC induction motors do not require sophisticated frequency and voltage control, an alternative to adjustable frequency controllers has been developed.

An alternate approach to the adjustable frequency controller is the soft starter. Soft starters operate using the principal of phase control whereby the line currents supplied to the AC induction motor are controlled by means of anti-parallel thyristor switches. Soft starters offer two major benefits for a user. First, use of a soft starter reduces the motor torque pulsation at startup of the AC induction motor which, in turn, results in less mechanical strain on the load. Second, use of a soft starter reduces the motor inrush current at startup of the AC induction motor which, in turn, places less stress on upstream electrical systems.

Typically, the anti-parallel thyristor switches are provided in each supply line. However, with delta motors, the anti-parallel thyristor switches may be provided inside the delta. Positioning the anti-parallel thyristor switches within the delta allows for use of smaller electrical components since the phase current magnitudes are less than the line current magnitudes.

In order to start an AC induction motor using a soft starter, two alternate types of control algorithms are used. In alpha control, the thyristor switches are sequentially fired at a certain angle, alpha (α), after the zero crossing times of the corresponding phase voltages. In gamma control, the thyristor switches are sequentially fired at a certain angle, gamma (γ), after the zero crossing times of the corresponding phase currents. It has been found that alpha control is more stable than gamma control in starting and bringing the AC induction motor to near operating speed because the zero crossings of the phase voltage are fixed while the zero crossings of the phase current move as the speed of the AC induction motor changes.

In addition, if gamma control is used during such period, greater current oscillation occurs which, in turn, leads to greater torque oscillation. More specifically, torque oscillation when the motor is stalled or when accelerating up to speed generally is caused by a DC component in the motor current. One of the principle advantages of utilizing a soft starter is that by applying the voltage to the motor slowly, these DC offset currents can be eliminated. However, it is possible to re-introduce DC currents when using gamma control. For example, utilizing a constant gamma when the integral of the motor phase current for the positive half cycle differs from the integral of the motor phase current for the negative half cycle, produces a DC component. In fact, if a disturbance introduces a slight asymmetry in firing of the thyristor switches, gamma control will exhibit a tendency to amplify the asymmetry, since an early current zero crossing will cause the next thyristor firing to occur earlier with respect to the terminal voltage. This effect is more pronounced when the soft starter is positioned inside the delta because the thyristor switches directly control phase voltage, that the current through the thyristor switches reflects the actual power of the motor (mostly inductive during acceleration), whereas line current control interposes a 30 degree angle between motor terminal voltage and line current being controlled.

Alternatively, gamma control is preferred once the motor has accelerated to the point where its speed is above the breakdown speed, in other words, the speed at which the slope of the speed/torque curve is zero. This is due to the fact that the motor's torque production is a delayed function of speed. Therefore, as the motor accelerates to full speed, it overshoots before the electrical torque balances the mechanical load. Once the motor overshoots, the electrical torque eventually becomes negative and the speed undershoots its steady state value causing torque to become positive again. It may take several cycles of this oscillation to reach steady state when the terminal voltage on the motor is fixed. However, when gamma control is used, the changes in motor power factor caused by the speed changes result in changes in current zero cross time which, in turn, affect the firing point for the next half cycle of current. This results in slight cycle to cycle changes in terminal voltage. These changes have a damping effect on the oscillations described above. In fact, in many cases, the oscillations are damped within a complete cycle.

Therefore, it is a primary object and feature of the present invention to provide an improved method for controlling the starting of an AC induction motor.

It is a further object and feature of the present invention to provide a method for controlling the starting of an AC induction in a stable manner.

It is still a further object and feature of the present invention to provide a method for controlling the starting an AC induction motor which utilizes smaller and less expensive components.

In accordance with the present invention, a method is provided for controlling a three phase, AC induction motor having three or more windings and six terminals. Each terminal is connectable to an AC input source for providing voltage and current to the AC induction motor. The windings of the AC induction motor are connected in a delta configuration. Each winding has a corresponding thyristor switch connected in series therewith such that each winding and thyristor switch combination is connected between two corresponding terminals of the AC induction motor. The method includes the steps of initially firing each thyristor switch to bring the AC induction motor toward full operating speed at a predetermined, initial alpha firing angle after occurrence of zero volts supplied by the AC input source. A new alpha firing angle is calculated and each thyristor switch is sequentially fired at the new alpha firing angle after occurrence of zero volts supplied by the AC input source to bring the AC induction motor toward full operating speed. Thereafter, each thyristor switch is sequentially fired at an initial gamma firing angle after occurrence of zero supply current therethrough to bring the AC induction motor toward full operating speed.

It is contemplated that the method includes the additional steps of providing the new alpha firing angle as the initial alpha firing angle and repeating for a predetermined time period the steps of calculating a new alpha firing angle and sequentially firing each thyristor switch. The new alpha firing angle may be calculated according to the expression:

α_(i)=−s*t+k

wherein α_(i) is the new alpha firing angle; s and k are constants; and t is the elapsed time from the initial firing of each thyristor switch.

Upon completion of the predetermined time period, each thyristor switch is sequentially fired at a user selected alpha firing angle after occurrence of zero volts supplied by the AC input source. The step of sequentially firing each thyristor switch at a user selected alpha firing angle is repeated for a second predetermined time period. Alternatively, motor current in the AC induction motor is monitored and if the motor current is greater than a predetermined value, the step of sequentially firing each thyristor switch at a user selected alpha firing angle is repeated

It is contemplated to monitor motor current in the AC induction motor and if the motor current is greater than a predetermined value, execute the additional steps of providing the new alpha firing angle as the initial alpha firing angle and repeating the steps of calculating a new alpha firing angle and sequentially firing each thyristor switch at the new alpha firing angle. The new alpha firing angle is calculated according to the expression:

α_(i)=α_(i−1)+k*(I−I_(lim))

wherein α_(i) is the new firing angle; α_(i−1) is the initial firing angle; k is a proportional gain; I is the integral of the motor current over the previous conduction interval; and I_(lim) is a user selected pre-set current limit. However, the absolute value of the expression k*(I−I_(lim)) is compared to a limit prior to calculating the new alpha firing angle and replaced with the limit in the expression to calculate the new alpha firing angle if the absolute value of the expression k*(I−I_(lim)) is greater than the limit.

The operating speed of the AC induction motor may be monitored and, if the operating speed is less than the full speed of the AC induction motor, the additional steps of calculating a new gamma firing angle; sequentially firing each thyristor switch at the new gamma firing angle after occurrence of zero supply current therethrough; providing the new gamma firing angle as the initial gamma firing angle; and returning to the step of calculating a new gamma firing angle are performed. The new gamma firing angle is calculated according to the expression:

γ_(i)=Y_(i−1)+k*(I−I_(lim))

wherein γ_(i) is the new gamma firing angle; γ_(i−1) is the initial gamma firing angle; k is a proportional gain; I is the integral of the motor current over the previous conduction interval; and I_(lim) is a user selected pre-set current limit. The absolute value of the expression k*(I−I_(lim)) is compared to a gamma limit prior to calculating the new gamma firing angle and is replaced with the gamma limit in the expression to calculate the new gamma firing angle if the absolute value of the expression k*(I−I_(lim)) is greater than the gamma limit.

Bypass contactors are provided in parallel across corresponding thryristor switches. The operating speed of the AC induction motor is monitored and the bypass contactors are closed in response to the AC induction motor operating at a predetermined operating speed.

In accordance with a further aspect of the present invention, a method is provided for controlling a three phase, AC induction motor having at least three windings and six terminals. Each terminal is connectable to an AC input source for providing voltage and current to the AC induction motor. The windings of the AC induction motor are connected in a delta configuration. Each winding has a corresponding thyristor switch connected in series therewith such that each winding and thyristor switch combination is connected between two corresponding terminals of the AC induction motor. The method includes the steps of firing each thyristor switch at an alpha firing angle after occurrence of zero volts supplied by the AC input source. The alpha firing angle is sequentially reduced and each thyristor switch is repeatedly fired at each reduced alpha firing angle after occurrence of zero volts supplied by the AC input source. The firing of each thyristor switch at the reduced alpha firing angles is stopped and, thereafter, each thyristor switch is sequentially fired at an initial gamma firing angle after occurrence of zero supply current therethrough.

Prior to stopping the firing of each thyristor switch, the additional steps of sequentially firing each thyristor switch at a user selected alpha firing angle after occurrence of zero volts supplied by the AC input source; and repeating the step of sequentially firing each thyristor switch at a user selected alpha firing angle for a predetermined time period are performed Alternatively, the motor current in the AC induction motor is monitored and, if the motor current is greater than a predetermined value, the step of sequentially firing each thyristor switch at a user selected alpha firing angle is repeated.

The alpha firing angle is calculated according to the expression:

α_(i)=α_(i−1)+k*(I−I_(lim))

wherein α_(i) is the calculated alpha firing angle; α_(i−1) is the alpha firing angle; k is a proportional gain; I is the integral of the motor current over the previous conduction interval; and I_(lim) is a user selected pre-set current limit. However, the absolute value of the expression k*(I−I_(lim)) is compared to a limit prior to calculating the new alpha firing angle and replaced with the limit in the expression to calculate the new alpha firing angle if the absolute value of the expression k*(I−I_(lim)) is greater than the limit.

The operating speed of the AC induction motor may be monitored and, if the operating speed is less than the full speed of the AC induction motor, the additional steps of calculating a new gamma firing angle; sequentially firing each thyristor switch at the new gamma firing angle after occurrence of zero supply current therethrough; providing the new gamma firing angle as the initial gamma firing angle; and returning to the step of calculating a new gamma firing angle are performed. The new gamma firing angle is calculated according to the expression:

γ_(i)=γ_(i−)+k*(I−I_(lim))

wherein γ_(i) is the new gamma firing angle; γ_(i−1) is the initial gamma firing angle; k is a proportional gain; I is the integral of the motor current over the previous conduction interval; and I_(lim) is a user selected pre-set current limit. The absolute value of the expression k*(I−I_(lim)) is compared to a gamma limit prior to calculating the new gamma firing angle and is replaced with the gamma limit in the expression to calculate the new gamma firing angle if the absolute value of the expression k*(I−I_(lim)) is greater than the gamma limit.

Bypass contactors are provided in parallel across corresponding thryristor switches. The operating speed of the AC induction motor is monitored and the bypass contactors are closed in response to the AC induction motor operating at a predetermined operating speed.

In accordance with a still further aspect of the present invention, a method is provided for controlling a three phase, AC induction motor having at least three windings and six terminals. Each terminal is connectable to an AC input source for providing voltage and current to the AC induction motor. The windings of the AC induction motor are connected in a delta configuration. Each winding has a corresponding thyristor switch connected in series therewith such that each winding and thyristor switch combination is connected between two corresponding terminals of the AC induction motor. The method includes the steps of firing initially firing each thyristor switch at a predetermined, initial alpha firing angle after occurrence of zero volts supplied by the AC input source. A new alpha firing angle is calculated and each thyristor switch is sequentially fired at the new alpha firing angle after occurrence of zero volts supplied by the AC input source. Thereafter, each thyristor switch is sequentially fired at a user selected alpha firing angle after occurrence of zero volts supplied by the AC input source, and then each thyristor switch is sequentially fired at an initial gamma firing angle after occurrence of zero supply current therethrough. Bypass contactors are provided in parallel across corresponding thryristor switches. The operating speed of the AC induction motor is monitored and the bypass contactors are closed in response to the AC induction motor operating at a predetermined operating speed. If the operating speed is less than the operating speed of the AC induction motor, the additional steps of calculating a new gamma firing angle; sequentially firing each thyristor switch at the new gamma firing angle after occurrence of zero supply current therethrough; providing the new gamma firing angle as the initial gamma firing angle; and returning to the step of calculating a new gamma firing angle are performed.

It is contemplated to provide the new alpha firing angle as the initial alpha firing angle and to repeat, for a predetermined time period, the steps of calculating a new alpha firing angle and sequentially firing each thyristor switch. Upon completion of the predetermined time period, the step of sequentially firing each thyristor switch at a user selected alpha firing angle after occurrence of zero volts supplied by the AC input source is performed and the step of sequentially firing each thyristor switch at a user selected alpha firing angle is performed for a second predetermined time period.

BRIEF DESCRIPTION OF THE DRAWINGS

The drawings furnished herewith illustrate a preferred construction of the present invention in which the above advantages and features are clearly disclosed as well as others which will be readily understood from the following description of the illustrated embodiment.

In the drawings:

FIG. 1 is a schematic view of a motor system for executing a method in accordance with the present invention;

FIGS. 2a and 2 b are graphical representations of the voltage across and the current through a pair of anti-parallel thyristor switches in FIG. 1 as a function of time,

FIG. 3 is a flow chart of computer executable instructions for executing a first embodiment of the method of the present invention;

FIG. 4 is a flow chart of computer executable instructions for executing a second embodiment of the method of the present invention; and

FIG. 5 is a flow chart of computer executable instructions for executing a third embodiment of the method of the present invention.

DETAILED DESCRIPTION OF THE DRAWINGS

Referring to FIG. 1, a motor system for executing a method in accordance with the present invention is generally designated by the reference number 10. Motor control system 10 includes an AC induction motor 12 having terminals 13 a-f. Terminals 13 a, 13 b and 13 c of AC induction motor 12 are coupled to a three phase AC source 18 by supply lines 16 a, 16 b and 16 c. As is conventional, AC source 18 provides line voltages V_(A), V_(B) and V_(C) and line currents I_(A), I_(B) and I_(C) through corresponding supply lines 16 a, 16 b and 16 c to AC induction motor 12.

In the preferred embodiment, AC induction motor 12 is a delta motor and soft starter 14 is applied in series with windings 20 a, 20 b and 20 c of AC induction motor 12. Soft starter 14 includes anti-parallel silicon controlled rectifiers (SCRs) or thyristor switches 22 a, 22 b, and 22 c which are connected in series with corresponding windings 20 a, 20 b and 20 c of AC induction motor 12. Each thyristor switch 22 a, 22 b and 22 c consists of a pair of inversely connected SCRs used to control the phase voltages V_(a), V_(b) and V_(c) across and phase currents I_(a), I_(b) and I_(c) through windings 20 a, 20 b and 20 c of AC induction motor 12. Bypass contactors 24 a, 24 b and 24 c are connected in parallel to corresponding thyristor switches 22 a, 22 b and 22 c, respectively, for reasons hereinafter described.

The phase voltages V_(a), V_(b) and V_(c), across and phase currents I_(a), I_(b) and I_(c) through windings 20 a, 20 b and 20 c of AC induction motor 12 are identical but for being 120° out of phase with each other. Further, phase currents I_(a), I_(b) and I_(c) lag the phase voltages V_(a), V_(b) and V_(c) by an angle θ corresponding to the power factor of AC induction motor 12. By way of example, referring to FIGS. 2a and 2 b, phase voltage V_(a) across winding 20 a is compared to the phase current I_(a) and terminal voltage V_(T) which is the difference in potential between terminals 13 d and 13 b of AC induction motor 12. As is known, the waveform of terminal voltage V_(T) is sinusoidal. The phase voltage V_(a) is generally identical to the terminal voltage V_(T) except during a small, non-conducting time or notch having a duration γ which is introduced into each half cycle of terminal voltage V_(T). Notch γ is introduced into terminal voltage V_(T) each time phase current I_(a) falls to zero. Phase current I_(a) remains at zero until the end of notch γ at which time phase current I_(a) continues a pulsating waveform.

The phase current I_(a) is controlled by the duration of notch γ. During notch γ, thyristor switch 22 a which interconnects motor terminal 13 a and winding 20 a operates as an open circuit so that instead of observing sinusoidal terminal voltage V_(T) between motor terminals 13 d and 13 b, an internal motor generated back EMF voltage may be seen. The back EMF voltage is generally equal to the terminal voltage V_(T) minus the voltage drop V_(ad) across thyristor switch 22 a.

In accordance with the present invention, three alternate approaches for starting and bringing the AC induction motor to its full operating speed are provided. Referring to FIG. 3, in the first approach generally designated by the reference numeral 28, thyristor switches 22 a, 22 b and 22 c are fired (turned on) after a user selected alpha firing angle a after the initial zero voltage crosses of corresponding terminal voltages V_(T). In the preferred embodiment, the user selected alpha firing angle a is generally equally to 135°, block 30. However, other values for the user selected alpha firing angle α are contemplated as being within the scope of the present invention.

Thereafter, the alpha firing angle α is reduced by a predetermined amount and thyristor switches 22 a, 22 b and 22 c are fired again at the reduced alpha firing angle α after the next zero voltage crosses of corresponding terminal voltages V_(T). In the preferred embodiment, the reduced or new alpha firing angle is calculated according to the expression:

α_(i)=−16*t+135

wherein α_(i) is the new alpha firing angle; “−16” and “135” are constants which may be varied according to the application; and I is the elapsed time from the initial firing of thyristor switches 22 a, 22 b and 22 c, block 32. The process is repeated for a predetermined period of time, e.g. 2.5 seconds, block 34.

Upon completion of the predetermined period of time, the alpha firing angle α is maintained at a selected alpha firing angle α, block 36, and thyristor switches 22 a, 22 b and 22 c are subsequently fired at such alpha firing angle α after each zero voltage crosses of corresponding terminal voltages V_(T) for a second predetermined time period, e.g. 8 seconds, block 38. Upon completion of the second predetermined time period, the control methodology is changed from alpha control to gamma control, block 40.

In gamma control, the phase currents I_(a), I_(b) and I_(c) of AC induction motor 12 are maintained by maintaining the half cycle time of thyristor switches 22 a, 22 b and 22 c such that the duration of notch γ is maintained until AC induction motor 12 is at full operating speed, block 42. In the preferred embodiment, the duration of notch γ is maintained at 10°, block 40, but other angles are contemplated as being within the scope of the present invention. Full operating speed of AC induction motor 12 may be determined in any conventional manner. For example, by maintaining the phase currents I_(a), I_(b) and I_(c) of AC induction motor 12 at a predetermined level over a predetermined period of time, the angle θ, FIGS. 2a and 2 b, of the power factor of AC induction motor 12 is reduced and the back EMF voltages approach the terminal voltages V_(T). Once the back EMF voltages exceed a predetermined value, AC induction motor 12 is at full operating speed and corresponding bypass contactors 24 a, 24 b and 24 c are sequentially closed, block 44, such that windings 20 a, 20 b and 20 c are connected directly between corresponding pairs of motor terminals 13 a, 13 b and 13 c.

Referring to FIG. 4, in the second approach generally designated by the reference numeral 46, thyristor switches 22 a, 22 b and 22 c are fired after a user selected alpha firing angle α, i.e. 135°, block 48, after the initial zero voltage crosses of corresponding terminal voltages V_(T). Thereafter, the alpha firing angle α is reduced and thyristor switches 22 a, 22 b and 22 c are fired again at the reduced alpha firing angle α after the next zero voltage crosses of corresponding terminal voltages V_(T). As heretofore described, in the preferred embodiment, the reduced or new alpha firing angle is calculated according to the expression:

α_(i)=−16*t+135

wherein α_(i) is the new alpha firing angle; “−16” and “135” are constants which may be varied according to the application; and t is the elapsed time from the initial firing of thyristor switches 22 a, 22 b and 22 c, block 50. The process is repeated for a predetermined period of time, e.g. 2.5 seconds, block 52.

Upon completion of the predetermined period of time, the alpha firing angle α is maintained, block 54, at a selected alpha firing angle α, e.g. 95°, and thyristor switches 22 a, 22 b and 22 c are subsequently fired at such alpha firing angle α after each zero voltage crosses of corresponding terminal voltages V_(T).

As thyristor switches 22 a, 22 b and 22 c are subsequently fired at alpha firing angle α, the current limit I_(lim) of AC induction motor 12 is determined. In the preferred embodiment, current limit is calculated according to the expression: I_(lim) = ∫_(FT)^(NST)i(t)t

wherein I_(lim) is the current limit, FT is the initial firing time of the thyristor switches; NST is the next stopping time of the thyristor switches; i(t) is the instantaneous phase current; and, in the preferred embodiment, 2.5 seconds<t<3.5 seconds, block 54.

Thereafter, the phase currents I_(a), I_(b) and I_(c) of AC induction motor 12 are determined according to the expression: I = ∫_(FT)^(NST)i(t)t

wherein I is the integral of the motor current over the previous conduction interval; FT is the initial firing time of the thyristor switches; NST is the next stopping time of the thyristor switches; and i(t) is the instanteous phase current, block 56.

As long as the instantaneous phase currents I_(a), I_(b) and I_(c) of AC induction motor 12 are greater than a predetermined percentage of the current limit I_(lim) of AC induction motor 12, e.g. 0.8*I_(lim), the process is repeated and thyristor switches 22 a, 22 b and 22 c are fired again at the selected alpha firing angle α, block 58. If the phase currents I_(a), I_(b) and I_(c) of AC induction motor 12 drop below the predetermined percentage of the current limit I_(lim), the control methodology is changed from alpha control to gamma control, block 60.

As heretofore described with respect to the first approach, in gamma control, the phase currents I_(a), I_(b) and I_(c) of AC induction motor 12 are maintained by maintaining the half cycle time of thyristor switches 22 a, 22 b and 22 c such that the duration of notch γ is maintained until AC induction motor 12 is at full operating speed, block 62. In the preferred embodiment, the duration of notch γ is maintained at 10°, block 60, but other angles are contemplated as being within the scope of the present invention.

Upon a determination that the AC induction motor 12 is at full operating speed, bypass contactors 24 a, 24 b and 24 c are sequentially closed, block 64, such that windings 20 a, 20 b and 20 c are connected directly between corresponding pairs of motor terminals 13 a, 13 b and 13 c.

Referring to FIG. 5, in the third approach generally designated by the reference numeral 66, thyristor switches 22 a, 22 b and 22 c are initially fired at a user selected alpha firing angle α, i.e. 135°, block 68, after the initial zero voltage crosses of corresponding terminal voltages V_(T). Thereafter, a new alpha firing angle α is calculated based upon the error between the integration of the phase currents I_(a), I_(b) and I_(c) of AC induction motor 12 and the current limit I_(lim) of AC induction motor 12, block 70. In the preferred embodiment, current limit Ii is a known constant, block 68, and the phase currents I_(a), I_(b) and I_(c) of AC induction motor 12 are determined according to the expression: I = ∫_(FT)^(NST)i(t)t

wherein I is the-integral of the motor current over the previous conduction interval; FT is the initial firing time of the thyristor switches; NST is the next stopping time of the thyristor switches; and i(t) is the instantaneous phase current, block 70.

The new alpha firing angle is reduced according to the expression:

α_(i)=α_(i−1)+k*(I−I_(lim))

wherein α_(i) is the new alpha firing angle; α_(i−1) is the prior alpha firing angle; k is a proportional gain; I is the integral of the motor current over the various conduction interval; and I_(lim) is a user selected pre-set current limit, block 72. To maintain the stability of AC induction motor 12, it is contemplated that the maximum absolute value of the expression k*(I−I_(lim)) be limited to 0.25°, block 72. Thyristor switches 22 a, 22 b and 22 c are fired again at the new alpha firing angle α_(i) after the next zero voltage crosses of corresponding terminal voltages V_(T) and the process is repeated until the phase currents I_(a), I_(b) and I_(c) of AC induction motor 12 drop below a predetermined percentage of the current limit I_(lim), e.g. 0.8*I_(lim), indicating that the AC induction motor 12 is approaching fill operating speed, block 74. Once the phase currents I_(a), I_(b) and I_(c) of AC induction motor 12 drop below the predetermined percentage of the current limit I_(lim), the control methodology is changed from alpha control to gamma control, block 76.

In gamma control, gamma is initialized, block 77, according to the expression:

γ_(i−1)=2*α_(i)−1800°

wherein γ_(i−1) is the initial gamma firing angle and α_(i) is the new alpha firing angle as heretofore described; and the phase currents I_(a), I_(b) and I_(c) are gradually increased over a period of time. In order to increase the phase currents I_(a), I_(b) and I_(c) applied to AC induction motor 12, the half cycle time of thyristor switches 22 a, 22 b and 22 c must be increased. In order to gradually increase the half cycle time of the thyristor switches 22 a, 22 b and 22 c, the duration of notch γ in the voltage waveforms V_(T) is reduced with each subsequent firing thyristor switches 22 a, 22 b and 22 c according to expression:

γ_(i)=γ_(i−1)+k*(I−I_(lim))

wherein γ, is the new gamma firing angle; γ_(i−1) is the initial gamma firing angle; k is a proportional gain; I is the integral of the motor current over the previous conduction interval; and I_(lim) is a user selected pre-set current limit, block 78. To maintain the stability of AC induction motor 12, it is contemplated that the maximum absolute value of the expression k*(I−I_(lim)) be limited to 0.25°, block 78. Thyristor switches 22 a, 22 b and 22 c are fired again at the new gamma firing angle γ_(i) after the next zero current crosses of corresponding phase currents I_(a), I_(b) and I_(c) and the process is repeated until the AC induction motor 12 is at full operating speed, block 80. Once the AC induction motor 12 is at full operating speed, bypass contactors 24 a, 24 b and 24 c are sequentially closed, block 82, such that windings 20 a, 20 b and 20 c are connected directly between corresponding pairs of motor terminals 13 a, 13 b and 13 c.

In order for soft starter 14 to function as heretofore described, it is contemplated that a microprocessor (not pictured) carry out a number of predetermined functions which are incorporated into computer executable instructions, FIGS. 3-5. It should be understood that while these functions are described as being implemented in software, it is contemplated that the functions could be implemented in discreet solid state hardware, as well as, the combination of solid state hardware and software.

It is contemplated that in order to effectuate the methods of the present invention, the microprocessor will include a plurality of inputs corresponding to selected parameters heretofore described. For example, such inputs include line voltages V_(A), V_(B) and V_(C); line currents I_(A), I_(B) and I_(C); phase voltages V_(a), V_(b) and V_(c); and phase currents I_(a), I_(b) and I_(c). Further, it is contemplated that the microprocessor generate a plurality of output signals which correspond to selected instructions heretofore described. For example, referring back to FIG. 1, microprocessor generates firing signals S_(A), S_(B) and S_(C) to fire corresponding thyristor switches 22 a, 22 b, and 22 c, respectively, at appropriate times, as heretofore described. Similarly, microprocessor generates firing signals B_(A), B_(B) and B_(C) to close corresponding bypass contactors 24 a, 24 b, and 24 c, respectively, at appropriate times, as heretofore described.

Various modes of carrying out the invention are contemplated as being within the scope of the following claims particularly pointing out and distinctly claiming the subject matter which is regarded as the invention. 

We claim:
 1. A method for controlling a three phase, AC induction motor having at least three windings and six terminals wherein three of the terminals are connectable to an AC input source for providing voltage and current to the AC induction motor and wherein the windings are connected in a delta configuration and each winding has a corresponding thyristor switch connected in series therewith such that each winding and thyristor switch combination is connected between two corresponding terminals of the AC induction motor, the method comprising the steps of: initially firing each thyristor switch at a predetermined, initial alpha firing angle after occurrence of zero volts supplied by the AC input source; calculating a new alpha firing angle; sequentially firing each thyristor switch at the new alpha firing angle after occurrence of zero volts supplied by the AC input source; and switching to gamma control and sequentially firing each thyristor switch at an initial gamma firing angle after occurrence of zero supply current therethrough.
 2. The method of claim 1 further comprising the steps of: providing the new alpha firing angle as the initial alpha firing angle; and repeating for a predetermined time period the steps of calculating a new alpha firing angle and sequentially firing each thyristor switch.
 3. The method of claim 2 wherein the new alpha firing angle is calculated according to the expression: α_(i)=−s*t+k wherein α_(i) is the new alpha firing angle; s and k are constants; and t is the elapsed time from the initial firing of each thyristor switch.
 4. The method of claim 3 further comprising the steps of: upon completion of the predetermined time period, sequentially firing each thyristor switch at a user selected alpha firing angle after occurrence of zero volts supplied by the AC input source; and repeating the step of sequentially firing each thyristor switch at a user selected alpha firing angle for a second predetermined time period.
 5. The method of claim 3 further comprising the steps of: upon completion of the predetermined time period, sequentially firing each thyristor switch at a user selected alpha firing angle after occurrence of zero volts supplied by the AC input source; and monitoring motor current in the AC induction motor and if the motor current is greater than a predetermined value, repeating the step of sequentially firing each thyristor switch at a user selected alpha firing angle.
 6. The method of claim 1 wherein prior to switching to gamma control, executing the additional steps of: monitoring motor current in the AC induction motor and if the motor current is greater than a predetermined value, executing the additional steps of: providing the new alpha firing angle as the initial alpha firing angle; and repeating the steps of calculating a new alpha firing angle and sequentially firing each thyristor switch at the new alpha firing angle.
 7. The method of claim 6 wherein the new alpha firing angle is calculated according to the expression: α_(i)=α_(i−1)+k*(I−I_(lim)) wherein α_(i) is the new firing angle; α_(i−1) is the initial firing angle; k is a proportional gain; I is an integral of the motor current over a conduction time interval; and I_(lim) is a user selected pre-set current limit.
 8. The method of claim 7 comprising the additional steps of comparing the absolute value of the expression k*(I−I_(lim)) to a limit prior to calculating the new alpha firing angle and replacing the expression k*(I−I_(lim)) with the limit in the expression to calculate the new alpha firing angle if the absolute value of the expression k*(I−I_(lim)) is greater than the limit.
 9. The method of claim 8 wherein the step of switching to gamma control includes the additional steps of: monitoring the operating speed of the AC induction motor and if the operating speed is less than the full speed of the AC induction motor, executing the additional steps of: initializing the gamma firing angle; calculating a new gamma firing angle; sequentially firing each thyristor switch at the new gamma firing angle after occurrence of zero supply current therethrough; providing the new gamma firing angle as the initial gamma firing angle; and returning to the step of calculating a new gamma firing angle.
 10. The method of claim 9 wherein the new gamma firing angle is calculated according to the expression: γ_(i)=γ_(i−1)+k*(I−I_(lim)) wherein γ_(i) is the new gamma firing angle; γ_(i−1) is the initial gamma firing angle; k is a proportional gain; I is an integral of the motor current over a conduction time interval; and I_(lim) is a user selected pre-set current limit.
 11. The method of claim 10 comprising the additional steps of comparing the absolute value of the expression k*(I−I_(lim)) to a gamma limit prior to calculating the new gamma firing angle and replacing the expression k*(I−I_(lim)) with the gamma limit in the expression to calculate the new alpha firing angle if the absolute value of the expression k*(I−I_(lim)) is greater than the gamma limit.
 12. The method of claim 1 comprising the additional steps of: providing bypass contactors in parallel across corresponding thryristor switches; monitoring the operating speed of the AC induction motor; and closing the bypass contactors in response to the AC induction motor operating at a predetermined operating speed.
 13. A method for controlling a three phase, AC induction motor having at least three windings and six terminals wherein three of the terminals are connectable to an AC input source for providing voltage and current to the AC induction motor and wherein the windings are connected in a delta configuration and each winding has a corresponding thyristor switch connected in series therewith such that each winding and thyristor switch combination is connected between two corresponding terminals of the AC induction motor, the method comprising the steps of: firing each thyristor switch at an alpha firing angle after occurrence of zero volts supplied by the AC input source; sequentially reducing the alpha firing angle and repeatedly firing each thyristor switch at each reduced alpha firing angle after occurrence of zero volts supplied by the AC input source; ceasing the firing of each thyristor switch at each reduced alpha firing angle after occurrence of zero volts supplied by the AC input source; and switching to gamma control and sequentially firing each thyristor switch to bring the AC induction motor toward full operating speed at an initial gamma firing angle after occurrence of zero supply current therethrough.
 14. The method of claim 13 wherein prior to the step of ceasing the firing of each thyristor switch, executing the additional steps of: sequentially firing each thyristor switch at a user selected alpha firing angle after occurrence of zero volts supplied by the AC input source; and repeating the step of sequentially firing each thyristor switch at a user selected alpha firing angle for a predetermined time period.
 15. The method of claim 13 wherein prior to the step of ceasing the firing of each thyristor switch, executing the additional steps of: sequentially firing each thyristor switch at a user selected alpha firing angle after occurrence of zero volts supplied by the AC input source; and monitoring motor current in the AC induction motor and if the motor current is greater than a predetermined value, repeating the step of sequentially firing each thyristor switch at a user selected alpha firing angle.
 16. The method of claim 13 wherein the alpha firing angle is reduced according to the expression: α_(i)=α_(i−1)+k*(I−I_(lim)) wherein α_(i) is the reduced alpha firing angle; α_(i−1) is the alpha firing angle; k is a proportional gain; I is an integral of the motor current over a conduction time interval; and I_(lim) is a user selected pre-set current limit.
 17. The method of claim 16 comprising the additional steps of comparing the absolute value of the expression k*(I−I_(lim)) to a limit prior to calculating the new alpha firing angle and replacing the expression k*(I−I_(lim)) with the limit in the expression to calculate the new alpha firing angle if the absolute value of the expression k*(I−I_(lim)) is greater than the limit.
 18. The method of claim 17 wherein the step of switching to gamma control includes the additional steps of: monitoring the operating speed of the AC induction motor and if the operating speed is less than the full speed of the AC induction motor, executing the additional steps of: initializing the gamma firing angle; calculating a new gamma firing angle; sequentially firing each thyristor switch at the new gamma firing angle after occurrence of zero supply current therethrough; providing the new gamma firing angle as the initial gamma firing angle; and returning to the step of calculating a new gamma firing angle.
 19. The method of claim 18 wherein the new gamma firing angle is calculated according to the expression: γ_(i)=γ_(i−1)+k*(I−I_(lim)) wherein γ_(i) is the new gamma firing angle; γ_(i−1) is the initial gamma firing angle; k is a proportional gain; I is an integral of the motor current over a conduction time interval; and I_(lim) is a user selected pre-set current limit.
 20. The method of claim 19 comprising the additional steps of comparing the absolute value of the expression k*(I−I_(lim)) to a gamma limit prior to calculating the new gamma firing angle and replacing the expression k*(I−I_(lim)) with the gamma limit in the expression to calculate the new alpha firing angle if the absolute value of the expression k*(I−I_(lim)) is greater than the gamma limit.
 21. The method of claim 13 comprising the additional steps of: providing bypass contactors in parallel across corresponding thryristor switches; monitoring the operating speed of the AC induction motor; and closing the bypass contactors in response to the AC induction motor operating at a predetermined operating speed.
 22. A method for controlling a three phase, AC induction motor having at least three windings and six terminals wherein three of the terminals are connectable to an AC input source for providing voltage and current to the AC induction motor and wherein the windings are connected in a delta configuration and each winding has a corresponding thyristor switch connected in series therewith such that each winding and thyristor switch combination is connected between two corresponding terminals of the AC induction motor, the method comprising the steps of: initially firing each thyristor switch at a predetermined, initial alpha firing angle after occurrence of zero volts supplied by the AC input source; calculating a new alpha firing angle; sequentially firing each thyristor switch at the new alpha firing angle after occurrence of zero volts supplied by the AC input source; sequentially firing each thyristor switch at a user selected alpha firing angle after occurrence of zero volts supplied by the AC input source; switching to gamma control and sequentially firing each thyristor switch at an initial gamma firing angle after occurrence of zero supply current therethrough; providing bypass contactors in parallel across corresponding thryristor switches; monitoring the operating speed of the AC induction motor; and closing the bypass contactors in response to the AC induction motor operating at a predetermined operating speed.
 23. The method of claim 22 further comprising the steps of: monitoring the operating speed of the AC induction motor and if the operating speed is less than the full speed of the AC induction motor, executing the additional steps of: calculating a new gamma firing angle; sequentially firing each thyristor switch at the new gamma firing angle after occurrence of zero supply current therethrough; providing the new gamma firing angle as the initial gamma firing angle; and returning to the step of calculating a new gamma firing angle.
 24. The method of claim 22 further comprising the steps of: providing the new alpha firing angle as the initial alpha firing angle; and repeating for a predetermined time period the steps of calculating a new alpha firing angle and sequentially firing each thyristor switch.
 25. The method of claim 22 wherein upon completion of the predetermined time period, executing the step of sequentially firing each thyristor switch at a user selected alpha firing angle after occurrence of zero volts supplied by the AC input source and repeating the step of sequentially firing each thyristor switch at a user selected alpha firing angle for a second predetermined time period.
 26. The method of claim 22 wherein upon completion of the predetermined time period, executing the step of sequentially firing each thyristor switch at a user selected alpha firing angle after occurrence of zero volts supplied by the AC input source and monitoring motor current in the AC induction motor and if the motor current is greater than a predetermined value, repeating the step of sequentially firing each thyristor switch at a user selected alpha firing angle. 